The ability to deposit materials into high aspect ratio (AR) holes (holes with a depth to width ratio greater than one) on a semiconductor substrate is becoming increasingly more important in the semiconductor industry for back-end-of-line applications such as the filling of inter-level vias.
Prior techniques used to deposit materials into high AR holes include: chemical vapor deposition (CVD); electron cyclotron resonance chemical vapor deposition (ECR CVD); plasma enhanced chemical vapor deposition (PECVD) and rf sputtering.
CVD is undesirable for two reasons: (a) it is a relatively slow process; and (b) high temperature substrate curing is required following the deposition. High temperature curing is unfavorable for large scale integrated (LSI) components.
ECR CVD and PECVD have been introduced to lower the required curing temperatures. These techniques, however, are also undesirable because the films produced often contain hydrogen, thereby making them dimensionally unstable at elevated temperatures. Also, these techniques, like all CVD processes, are slow processes that restrain throughput.
Traditional rf sputtering does not fill high AR holes uniformly because there is no directionality to the deposition. The sputtered atoms in the plasma generated are primarily neutral (plasma referred to as having a low ionization ratio). The resultant random neutral atom impingment on the substrate causes sputtered material to build up along the walls of the holes faster than the bottom thereby producing a void in the filled hole (a condition referred to as angel wings) which makes contact to the filled hole unreliable. Also, the background gas (e.g. Argon) necessary to support the process: a) causes back scattering of the sputtered material which consequently reduces the transmission of this material to the substrate; and b) gets incorporated into the films. A further disadvantage to traditional rf sputtering is that it is difficult to obtain a plasma density high enough to sputter at energy efficient voltages for single substrate deposition.
The following three articles disclose some recent improvements to traditional rf sputtering: 1) Ono, Takahashi, Oda, and Matsuo, "Reactive Ion Stream Etching and Metallic Compound Deposition Using ECR Plasma Technology", Symposium on VLSI Technology. Digest of Technical Papers 1985 pp 84-85, Bus. Center Acad. Soc Japan (ONO); 2) Yamashita, "Fundamental characteristics of built-in high-frequency coil-type sputtering apparatus", J.Vac.Sci.Technol. A7(2), Mar/Apr 1989 pp 151-158 (YAMASHITA); and 3) Matsuoka and Ono, "Dense plasma production and film deposition by new high-rate sputtering using an electric mirror", J.Vac.Sci.Technol. A7(4), Jul/Aug 1989 pp 2652-2656 (MATSUOKA).
ONO discloses an ECR plasma generator adapted for depositing metals on a substrate by placing a cylindrical sputtering target around the plasma stream at the plasma extraction window. Sputtered particles are activated for film formation reaction in the high plasma density region of magnetron mode discharge and in the plasma stream. Film formation reaction is also enhanced by the moderate energy ion bombardment irradiated by the plasma stream. This system provides higher deposition rates than prior sputtering techniques, but it doesn't provide directionality to the deposition (due to a low ionization ratio) and therefore fails to provide a solution to the aforementioned angel wing problem.
YAMASHITA discloses a sputtering system with a high-frequency discharge coil placed between the target and the substrate holder. This configuration can produce a plasma with a variable ionization ratio of the sputtered atoms. High ionization ratio deposition is not practical with this apparatus, however, because secondary electrons generated by the ions impinge on the substrate and thereby cause excessive substrate heating. Furthermore, high deposition rates can only be achieved with this device when the deposition is dominated by neutral atoms. Depositions dominated by neutral atoms have no directionality and therefore cause angel winging in high AR holes.
MATSUOKO discloses a sputtering system using an electric mirror consisting of a planar target, a cylindrical target, a magnetic coil, and a substrate plate. High plasma densities (of primarily neutral atoms) in close proximity to the cylindrical target are obtained by this configuration, but like the ONO configuration, there is no directionality to the deposition and angel winging occurs.
An efficient, reliable system to deposit materials into high AR holes on semiconductor substrates which avoids the problems of the previously mentioned systems is highly desirable.